Soil respiration responses to grazing and climate change simulations across topographical gradients in northern Mongolia

Background/Question/Methods

Globally, soil respiration is one of the largest fluxes of carbon to the atmosphere and is known to be sensitive to climate change. The mean annual air temperature of northern Mongolia has increased by 1.8 °C over the last 40 years, greater than global average temperature increase; in addition, the area is susceptible to overgrazing. We conducted field experiments to study independent and combined impacts of topography, grazing and climate manipulations on soil effluxes in northern Mongolia. Fifteen replicate plots with ITEX-style open-top passive warming chambers and non-warmed control areas were installed across topographical gradients. Fall, winter and spring grazing was allowed on 9x4 m² area next to the year-round enclosed 9x9 m² area. A weekly watering treatment equal to 4.5 mm of rainfall per week was applied. Soil moisture and temperatures were monitored on daily basis and CO₂ effluxes were measured 3-5 times per growing season for 2009, 2010 and 2011. CO₂ efflux measured in the vegetated area is hereafter referred to as ecosystem respiration. In contrast, CO₂ efflux measurements made on the bare soil area of plots is referred as soil respiration.

Results/Conclusions

We found that local gradients or heterogeneity did not contribute substantially to variations in soil respiration, but that responses of different plant communities to climate change may be important for ecosystem respiration. Soil respiration responses to simulated climate change were similar in upper and lower slopes, which differed in nutrient and moisture availability, and plant communities. While soil respiration did not differ, ecosystem respiration responses to simulated climate change differed between the upper and the lower slopes, implying that the responses of vegetation to climate change should be integrated into carbon cycling. Our results also showed that light grazing may enhance soil respiration while it decreases ecosystem respiration, and its impact may not depend on climate change. Overall, our results indicated that soil moisture is the most important driving factor for carbon fluxes in this semi-arid ecosystem. Rather than warming, increased variability in precipitation may trigger greater changes in soil and ecosystem respiration in this ecosystem.